1. Field of the Invention
The present invention relates to a substrate processing apparatus for processing a substrate, such as a semiconductor wafer, a glass substrate for a liquid crystal display or a substrate for a plasma display panel (PDP), which has a generally circular configuration.
2. Description of Related Art
A semiconductor device production process generally involves the step of forming a minute pattern on a surface of a semiconductor wafer (hereinafter referred to simply as "wafer") by repeatedly subjecting the wafer to a film forming and etching operation. Since the surface of a wafer and the surface of a thin film formed on a wafer surface should be kept clean for successful micro-processing of the wafer, a wafer cleaning operation is performed as required. For example, after a thin film formed on the surface of a wafer is polished with an abrasive (slurry), the slurry remaining on the wafer surface should be removed.
One exemplary apparatus for cleaning a wafer has three retention rollers 201, 202 and 203 as shown in FIGS. 8 and 9.
FIGS. 8 and 9 are a plan view and a side view, respectively, of the apparatus.
A circular wafer W to be cleaned is maintained horizontally with its periphery abutting against the retention rollers 201 to 203. A scrub-cleaning member 205 having a cleaning disc brush 204 is disposed above the wafer W. The upper surface of the wafer W is scrub-cleaned by rotating the scrub-cleaning member 205 with a contact surface 206 of the cleaning brush 204 being in contact with the upper surface of the wafer W. Another scrub-cleaning member 208 having a cleaning disc brush 207 is disposed below the wafer W. The lower surface of the wafer W is scrub-cleaned by the scrub-cleaning member 208.
When the wafer W is cleaned by the scrub-cleaning members 205 and 208, the wafer W is rotated, for example, clockwise about the rotation center OW of the wafer W as seen in FIG. 8 by the retention roller 201 so that the entire upper and lower surfaces of the wafer W can be scrubbed with the cleaning brushes 204 and 207. More specifically, the apparatus includes a motor 209 for rotating the retention roller 201 as shown in FIG. 8. The rotation force of the motor 209 is transmitted to the retention roller 201, whereby the retention roller 201 is rotated, for example, counterclockwise. As a result, the wafer W having a radius r receives a rotative driving force f exerted along a common tangent line from the retention roller 201, so that a rotation torque f.times.r is applied to the wafer W to rotate the wafer W clockwise. At this time, the retention rollers 202 and 203 are rotated by the rotation of the wafer W.
With this arrangement, however, the overall wafer W receives a force (eccentric force) having the same magnitude and direction as the rotative driving force f. This results in unstable rotation of the wafer W.
The unstable rotation of the wafer W causes the wafer W to be heavily pressed against one of the retention rollers 201 to 203, whereby a greater friction resistance is generated between the wafer W and the retention roller 201 to 203. As a result, the wafer W or the retention roller 201 to 203 is worn out, so that a greater amount of dust is generated. In addition, the service life of the retention rollers 201 to 203 is shortened.